Metal-mediated viscosity reduction of fluids gelled with viscoelastic surfactants

ABSTRACT

Fluids viscosified with viscoelastic surfactants (VESs) may have their viscosities reduced (gels broken) by the direct or indirect action of a composition that contains at least one metal ion source and optionally at least one second source. An optional second source may be a chelating agent where at least one reducing agent source may be additionally optionally used. Another optional component with the metal ion source includes a second, different metal ion source. The breaking composition is believed to directly attack the VES itself, possibly by disaggregating or otherwise attacking the micellar structure of the VES-gelled fluid, and/or possibly by changing the chemical structure of the VES to give two or more products.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 11/145,630 filed Jun. 6, 2005, issued Sep. 29, 2009as U.S. Pat. No. 7,595,284, which in turn claims the benefit of U.S.Provisional Application No. 60/577,682 filed Aug. 20, 2004.

TECHNICAL FIELD

The present invention relates to gelled treatment fluids used duringhydrocarbon recovery operations, and more particularly relates, in oneembodiment, to methods of “breaking” or reducing the viscosity ofaqueous treatment fluids containing viscoelastic surfactant gellingagents used during hydrocarbon recovery operations.

BACKGROUND

Hydraulic fracturing is a method of using pump rate and hydraulicpressure to fracture or crack a subterranean formation in a process forimproving the recovery of hydrocarbons from the formation. Once thecrack or cracks are made, high permeability proppant, relative to theformation permeability, is pumped into the fracture to prop open thecrack. When the applied pump rates and pressures are reduced or removedfrom the formation, the crack or fracture cannot close or healcompletely because the high permeability proppant keeps the crack open.The propped crack or fracture provides a high permeability pathconnecting the producing wellbore to a larger formation area to enhancethe production of hydrocarbons.

The development of suitable fracturing fluids is a complex art becausethe fluids must simultaneously meet a number of conditions. For example,they must be stable at high temperatures and/or high pump rates andshear rates that can cause the fluids to degrade and prematurely settleout the proppant before the fracturing operation is complete. Variousfluids have been developed, but most commercially used fracturing fluidsare aqueous based liquids that have either been gelled or foamed. Whenthe fluids are gelled, typically a polymeric gelling agent, such as asolvatable polysaccharide, for example guar and derivatized guarpolysaccharides, is used. The thickened or gelled fluid helps keep theproppants within the fluid. Gelling can be accomplished or improved bythe use of crosslinking agents or crosslinkers that promote crosslinkingof the polymers together, thereby increasing the viscosity of the fluid.One of the more common crosslinked polymeric fluids is boratecrosslinked guar.

The recovery of fracturing fluids may be accomplished by reducing theviscosity of the fluid to a low value so that it may flow naturally fromthe formation under the influence of formation fluids. Crosslinked gelsgenerally require viscosity breakers to be injected to reduce theviscosity or “break” the gel. Enzymes, oxidizers, and acids are knownpolymer viscosity breakers. Enzymes are effective within a pH range,typically a 2.0 to 10.0 range, with increasing activity as the pH islowered towards neutral from a pH of 10.0. Most conventional boratecrosslinked fracturing fluids and breakers are designed from a fixedhigh crosslinked fluid pH value at ambient temperature and/or reservoirtemperature. Optimizing the pH for a borate crosslinked gel is importantto achieve proper crosslink stability and controlled enzyme breakeractivity.

While polymers have been used in the past as gelling agents infracturing fluids to carry or suspend solid particles as noted, suchpolymers require separate breaker compositions to be injected to reducethe viscosity. Further, such polymers tend to leave a coating on theproppant and a filter cake of dehydrated polymer on the fracture faceeven after the gelled fluid is broken. The coating and/or the filtercake may interfere with the functioning of the proppant. Studies havealso shown that “fish-eyes” and/or “microgels” present in some polymergelled carrier fluids will plug pore throats, leading to impairedleakoff and causing formation damage.

Recently it has been discovered that aqueous drilling and treatingfluids may be gelled or have their viscosity increased by the use ofnon-polymeric viscoelastic surfactants (VES). These VES materials areadvantageous over the use of polymer gelling agents in that they do notleave a filter cake on the formation face, do not coat the proppant orcreate microgels or “fish-eyes”, and have reduced potential for damagingthe formation relative to polymers. However, little progress has beenmade toward developing internal breaker systems for the non-polymericVES-based gelled fluids, that is, breaker systems that use products thatare incorporated and solubilized within the VES-gelled fluid that areactivated by downhole conditions that will allow a controlled rate ofgel viscosity reduction over a rather short period of time of 1 to 4hours or so similar to gel break times common for conventionalcrosslinked polymeric fluid systems. A challenge has been thatVES-gelled fluids are not comprised of polysaccharide polymers that areeasily degraded by use of enzymes or oxidizers, but are comprised ofsurfactants that associate and form viscous rod- or worm-shaped micellestructures. Conventional enzymes and oxidizers have not been found toact and degrade the surfactant molecules or the viscous micellestructures they form. It is still necessary, however, to provide somemechanism that uses internally solubilized breaker products that willbreak the viscosity of VES-gelled fluids.

It would be desirable if a viscosity breaking system could be devised tobreak the viscosity of fracturing and other well completion fluidsgelled with and composed of viscoelastic surfactants, and in particularbreak the viscosity relatively quickly.

SUMMARY

Accordingly, it is an object of the present invention to provide amethod for breaking the viscosity of aqueous treatment fluids gelledwith viscoelastic surfactants (VESs).

It is another object of the present invention to provide compositionsand methods for breaking VES-surfactant substrates fluids relativelyquickly.

Still another object of the invention is to provide methods and VESfluid compositions for breaking the viscosity of aqueous fluids gelledwith viscoelastic surfactants using readily available materials atrelatively inexpensive concentrations.

In carrying out these and other objects of the invention, there isprovided, in one form, a method for breaking viscosity of aqueous fluidsgelled with a viscoelastic surfactant (VES) that involves adding to anaqueous fluid gelled with at least one viscoelastic surfactant acomposition in an amount effective to reduce the viscosity of the gelledaqueous fluid. The composition includes at least one metal ion source.Optional components of the composition may include, but are notnecessarily limited to, a reducing agent source, and/or a chelatingagent, or may be a second metal ion source.

There is provided in another non-limiting embodiment herein a method forbreaking viscosity of aqueous fluids gelled with a VES involving addingto an aqueous fluid gelled with at least one viscoelastic surfactant acomposition in an amount effective to reduce the viscosity of the gelledaqueous fluid, The composition includes at least one metal ion source,at least one chelating agent and at least one reducing agent source. Thecomposition may reduce the viscosity of the gelled aqueous fluid by amechanism including, but not necessarily limited to, disaggregating amicelle structure of the VES, rearranging a micelle structure of theVES, chemically altering an effective amount of the VES, andcombinations thereof.

In another, alternate embodiment, there is provided an aqueous fluidthat includes water; at least one viscoelastic surfactant (VES) in anamount effective to increase the viscosity of the aqueous fluid and acomposition in an amount effective to reduce the viscosity of the gelledaqueous fluid. Again, the composition includes at least one metal ion.The optional components may be those described in the previous twoparagraphs above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the viscosity of a 3% bw KCl aqueous fluid gelledwith 4% bv WG-3L at 150° F. (66° C.) with no Fe⁺² transition metal, 200ppm Fe⁺², and 400 ppm Fe⁺² demonstrating that the transition metal alonemay break the gel;

FIG. 2 is a graph of viscosity of a 3% bw KCl aqueous fluid gelled with4% bv WG-3L at 150° F. (66° C.) with no metal or chelant compared withvarious levels of Fe⁺² transition metal with two different levels of twodifferent chelants, Na citrate and AM2-45;

FIG. 3 is a graph of viscosity of a 3% bw KCl aqueous fluid gelled with4% bv WG-3L at 150° F. (66° C.) using 200 ppm Fe⁺² transition metalalone compared with 20 ppm Fe⁺² and 10 pptg (1.2 kg/m³) ascorbatereducing agent showing a sharper breaking profile with the latter;

FIG. 4 is a graph of viscosity of a 3% bw KCl aqueous fluid gelled with4% bv WG-3L at 150° F. (66° C.) employing 20 ppm Fe⁺² and 2 gptg AM2-45giving no breaking, some breaking effect starting at 4 hours with 20 ppmFe⁺² and 10 pptg (1.2 kg/m³) ascorbate, contrasted with 20 ppm Fe⁺² and10 pptg (1.2 kg/m³) ascorbate together with 1 gptg AM2-45 chelant givingsharp breaking in about 0.5 hour;

FIG. 5 is a graph of viscosity of a 3% bw KCl aqueous fluid gelled with4% bv WG-3L at 150° F. (66° C.) with various levels of ascorbatereducing agent only to optimize the levels showing generally breakingincreased with increasing ascorbate levels until the last two tried: 3.5pptg (0.42 kg/m³), 7 pptg (0.84 kg/m³), 10.5 pptg (1.3 kg/m³), 14 pptg(1.7 kg/m³) and 17.5 pptg (2.1 kg/m³);

FIG. 6 is a graph of viscosity of a 3% bw KCl aqueous fluid gelled with4% bv WG-3L at 150° F. (66° C.) with various levels of ascorbatereducing agent and 1 gptg AM2-45 chelating agent demonstrating how theviscosity break time of a VES fluid may be adjusted by varying theamount of metal;

FIG. 7 is a graph of viscosity of a 3% bw KCl aqueous fluid gelled with4% bv WG-3L at 150° F. (66° C.) with 2 ppm Fe+3 alone and together withvarying amounts of a second metal ion showing breaking of the gel; and

FIG. 8 is a graph of viscosity of a 3% bw KCl aqueous fluid gelled with4% bv WG-3L at 150° F. (66° C.) demonstrating that an oxidizer (Napersulfate) together with a metal and optionally ascorbate reducingagent may also break a VES-gelled fluid.

DETAILED DESCRIPTION OF THE INVENTION

As noted, aqueous fluids gelled with viscoelastic surfactants aretypically used in wellbore completions, such as hydraulic fracturing,without the use of an internal breaker system, and typically rely onexternal downhole conditions for the VES-gelled fluid to break, such asdilution with reservoir brine or gel breaking through interaction withreservoir hydrocarbons during production of such reservoir fluids to thesurface. There are aqueous fluids gelled with viscoelastic surfactantsthat are known to be “broken” or have their viscosities reduced,although some of the known breaking methods utilize external clean-upfluids (such as pre- and post-flush fluids placed within the reservoirbefore and after well completion treatments, such as conventional gravelpacking and also “frac-packing”—hydraulic fracturing followed by gravelpacking treatment). There are other known methods, but they arerelatively slow—for instance the use of VES-gel breaking bacteria withfluid viscosity break times ranging from half a day up to 7 days. Therehas evolved in the stimulation fluid art an industry standard need for“quick gel break”, but for VES-gelled fluids this has been asubstantially challenging problem. There needs to be a method forbreaking VES-gelled fluids that is as easy, as quick, and as economic asbreaking conventional crosslinked polymer fluids.

A new method has been discovered to reduce the viscosity of aqueousfluids gelled with viscoelastic surfactants (i.e. surfactants thatdevelop viscosity in aqueous brines by formation of rod- or worm-shapedmicelle structures). The improvement will allow relatively very quickbreaks, such as within 1 to 12 hours, compared to the current technologyof using bacteria to break VES which takes at least 12 or more hours,and more typically 4 to 7 days. The breaker components of this inventioncan be added to the gel and put into solution during a VES-gel treatmentor the components can be used separately, if needed, as an externalbreaker solution to remove VES gelled fluids already placed downhole.

The method employs at least one metal ion source as a breaker component.Without wanting to limit the invention to any supposed theory ormechanism, the alteration that occurs in breaking the VES gel isbelieved to be transition metal-mediated and/or transitionmetal-catalyzed. The terms “metal-mediated” and “metal-catalyzed” areused herein as equivalent terms, and mean that a transition metal isneeded for the reaction or sequence of reactions to occur, whether ornot the exact mechanism is catalytic.

The terms “altered” and “alteration” are used herein to mean any changeto the VES compound where it can no longer form, maintain or sustainviscous micelle structures. Thus, “altered” or “alteration” may include,but are not necessarily limited to: (i) a rearrangement of bonds on theVES, (ii) an addition to the VES (such as hydrogen, water molecule,etc.) or (iii) an elimination (decomposition or degradation) of the VES,e.g. where the VES after alteration now equals two or more othercompounds.

The primary reaction that chemically alters the VES structure isbelieved to be a redox reaction, without necessarily being limited bythis explanation. That is, it is expected that both reduction andoxidation occur in the reaction. A “redox” reaction is defined herein tobe any reaction in which electrons are removed from one molecule or atomand given to another molecule or atom. In the processes describedherein, such redox reactions are transition metal-mediated.

In most cases in the methods described herein, the alteration thatoccurs is not complete; meaning not all of the VES (e.g. VES compoundssuch as Akzo Nobel Aromox APA-T) is altered; only a portion of themolecules has been altered. In practical terms, the metal-mediatedalteration results in a ratio of altered to unaltered VES molecules.That is, typically a “broken VES fluid” is composed of a ratio ofaltered to unaltered VES molecules.

The ratio or amount of altered to unaltered VES molecules that cause VESgel break appears to be based on one or more of the following factorsand possibly others:

-   -   a. less altered VES is required to break the gel as fluid        temperature increases;    -   b. more altered VES is required to break the gel as VES (such as        Aromox APA-T VES) loading increases;    -   c. more altered VES is required to break the gel when VES        counterions or stabilizing agents are used, including, but not        necessarily limited to, CaCl₂, CaBr₂, MgO, CaOH, NH₄Cl,        salicylate, naphthalene sulfonate, phthalate, and the like.

In most cases it appears the VES (compounds such as Aromox APA-T) ispredominantly altered into a non-VES surfactant compound or chemicalspecies, for instance, a surfactant species that is not able to formviscous micelles (elongated or work-like micelle structures) or itremains predominantly a surfactant that has lost the ability to form VESmicelles. These theories are based on preliminary investigating andevaluating of the “residual material” that is sometimes left as aseparate liquid phase after VES gel breaking occurs.

In some cases the altered VES may be the VES surfactant degraded to ahydrocarbon tail and a hydrophilic head. Thus, the term “decomposition”could be used for describing the breaking of the VES-gelled fluid, but“metal-mediated” and “alteration” of the VES are better terms forexplaining the breaking phenomenon that occurs. As mentioned above, inmost cases the VES compound is predominantly altered into a non-VES typesurfactant. However, it may be understood that the surfactant (orsurfactants or products) generated are not as soluble or as dispersiblein water. That is, it has been found that the surfactant character ofthe products is overall less hydrophilic, and/or theHydrophilic-Lipophilic Balance (HLB) appears to be altered, and the HLBnumber appears to be lower.

At this point it is still not clear what linkages or bonds are alteredin the primary reactions that occur, whether the alteration occurs onthe hydrocarbon tail or the surfactant head group. It is also uncertainwhat specific alterations occur, such as electron addition, electronremoval, hydrogenation (electron and proton addition), dehydrogenation(electron and proton removal), and the like during the metal-mediatedredox reactions. However, without being limited to any particulartheory, it is suspected that the head group is the component that ischemically altered or modified. It is possible the head group ismodified (by metal-mediated redox reactions) to have less solubilityand/or dispersability in water, particularly brine (salt) water that istypically used for hydraulic fracturing operations.

The altered VES species appears to associate with the unaltered VES andas the ratio of altered to unaltered VES increases, a point is reachedwhere the amount of altered VES present does not allow the unaltered VESsurfactant to remain organized in worm-like or rod shaped viscousmicelle structures, and thereby alters the micelle by rearrangement anda complete viscosity break is achieved. As long as the ratio of alteredto unaltered VES remains relatively low the viscosity break that occursis a uniphase fluid: a fluid that appears like water containingsurfactants that do not yield viscosity, do not phase separate from thewater, but give the water a slight color (such as straw yellow and lightamber in some non-limiting cases) and the broken fluid easily foams whenshaken in a bottle, and has a viscosity resembling water.

However, it has been observed that if the ratio of altered to unalteredVES becomes relatively high, such as when significant amounts of breakerproducts are used and very quick VES gel breaks are achieved, generatingrelatively high amounts of altered VES will result in the altered VES tophase out as a liquid from the water phase, and the unaltered VESportion also phases out with the altered portion. The phase separationseen from relatively fast VES gel break times appears to be due to anumber of factors including, but not necessarily limited to, theselisted which may act alone or in concert.

-   -   a. The amount of altered VES generated.    -   b. The apparent low HLB number of the altered VES species.    -   c. Due to the apparent low HLB number it appears the altered VES        wants to associate more with itself (like an oil) than with        water.    -   d. Low HLB number surfactants in general have less solubility        and/or dispersability in water, particularly in brine water        (i.e. water with dissolved salts present, such as KCl, NaCl,        CaCl₂, CaBr₂, etc.).    -   e. It also appears that the ratio of altered to unaltered VES        may come to a point where the amount of unaltered VES present is        not able to act as a hydrotrope and keep the low HLB number        surfactant in solution and/or dispersed in the water phase.    -   f. The unaltered VES phasing out with the altered VES surfactant        species may possibly be due to the over abundance of altered VES        surfactant species present combined with possibly having a        strong attraction and interaction of the hydrocarbon tails that        results in an oil-type break and surfactant liquid phasing out        of the water phase.    -   g. Lab tests have shown that the liquid surfactant layer that        may phase out with fast breaking fluid compositions when shaken        in a bottle with the mix water brine will temporarily disperse        within the mix water for several minutes to several hours,        depending on the ratio of altered to unaltered VES within the        fluid-liquid surfactant layer.

Solubilizing, dispersing, and/or stabilizing the altered and unalteredVES from phasing out of the water phase can be enhanced by the use ofsolvents and hydrotropes, such as: glycerol, ethylene glycol and otherglycols, methanol and other alcohols, ethylene glycol monobutyl etherand other glycol ethers, ethoxylated alcohols, alkyl glucosides, alkylaromatic sulfonates, and the like, and combinations thereof.Solubilizing additive packages can be formulated to have enhancedperformance compared to single component solvent or hydrotrope additiveuse. One preferred synergistic additive package art is disclosed in U.S.patent application Ser. No. 11/430,655 filed May 9, 2006, published Nov.16, 2006 as U.S. Patent Application Publication No. 2006/0258541 A1,incorporated herein by reference in its entirety.

The particular ratio of altered to unaltered VES appears to depend on anumber of factors, some of which may have been identified. The ratioseems to depend primarily on the amount of breaker products used, morespecifically the amount of both the reducing agent (if present) and themetal ions. The ratio appears to also depend on the fluid temperature,the type and amount of mix water salt VES loading, and the like, andcombinations thereof.

The alteration of the VES (such as Aromox APA-T) thus appears to bemetal-mediated or metal-catalyzed. That is, the reaction occurs due tothe presence of a transition metal, as seen in the Examples of all FIGS.herein. Transition metals work alone at high enough metalconcentrations, as seen in FIG. 1. Generally, transition metals workfaster (the breaking rate is enhanced) when they are complexed with achelant as seen in FIG. 2. The transition metal may also work faster(the breaking rate is enhanced) in the presence of a reducing agent, asshown in FIG. 3. It has been additionally discovered that the transitionmetal works significantly faster (the rate is significantly enhanced)with the synergistic combination of a chelant with reducing agent isused with the metal. This may be seen primarily in FIG. 4 that compareshow the combination of a chelant with a reducing agent gavesignificantly shorter break time, thus providing evidence of synergismwith such a combination. It has also been discovered that the rate canfurther be enhanced if more than one metal ion is used with thecombination of a chelant and a reducing agent, as shown in FIG. 7.Further it has been found that an oxidizer can be activated by a metaland reducing agent combination as seen in FIG. 8. The data in FIG. 8 mayshow that the metal may be a catalyst to ascorbate and persulfate togenerate free radical oxidation species, with such species then beingthe agents which act on altering the VES molecules. This data may thusshow that the metal ions do not always have to be the agent itselfacting on altering the VES directly, that is other redox alterationpathways that may be present that are metal-mediated, and which arewithin the methods and compositions herein.

The method thus employs a metal ion source. Surprisingly andunexpectedly, the use of a metal ion possibly acting as a catalyst, suchas ferrous iron, alone does not give any early or rapid breaking effectat concentrations up to 240 ppm, and the use of an organic redox agentalone, such as 10.0 to 20.0 pptg sodium ascorbate, does not give anyparticular breaking effect alone either, but the use of both componentstogether provide a full and complete and rapid breaking of the gel, andthe use of a metal ion source alone at a high enough concentration mayalso break the gel.

Two preferred, but non-limiting, components of the inventive compositioninclude ferrous chloride and sodium ascorbate. Controlled viscosityreduction rates can be achieved from 75° F. to about 300° F. (about 24°C. to about 149° C.). As noted, more than one transition metal can becombined and more than one redox and/or possiblyhydrogenation-dehydrogenation agents can be combined. More than onetransition metal ion refers to the combination of two or more differenttransition metal ions. The utilization of more than one transition metaland more than one redox agent and/or possiblyhydrogenation-dehydrogenation agent may have applications in controllingpossible secondary reactions, that is, further reactions involving theremaining or altered viscoelastic surfactant substrate and/or to thedegraded surfactant by-products. Besides the use of more than one metal,addition of other agents may also be employed that may influence theprimary reaction (viscosity reduction) and secondary reactions(alteration of by-products), with agents such as pH buffers, alcohols,amines, sugars, and the like, and mixtures thereof.

The use of the disclosed breaker system is ideal for controlledviscosity reduction of VES-based fracturing fluids. The breaking systemmay also be used for breaking gravel pack and loss circulation pillfluids composed of VES. This VES breaking method is a significantimprovement in that it gives breaking rates for VES based fluids thatthe industry is accustomed to with conventional polymer based fracturingfluids, such as borate crosslinked guar.

In one non-limiting embodiment of the invention, the compositions hereinwill directly degrade or digest the gel created by a VES in an aqueousfluid, and alternatively will reduce the viscosity of the gelled aqueousfluid either directly, or by disaggregation or rearrangement of the VESmicellar structure. However, the inventor does necessarily not want tobe limited to any particular mechanism.

The composition of this invention includes at least one metal ion sourcewhere the goal is to deliver at least one metal ion to the VES-gelledsystem. The metal ion may be selected from metals including, but notnecessarily limited to, Groups VA, VIA, VIIA, VIIIA, IB, IIB, IIIB andIVB of the Periodic Table (previous IUPAC American Group notation), suchas iron, copper, manganese, cobalt, zinc, nickel, vanadium, platinum,tin, aluminum, molybdenum, platinum, palladium, and mixtures thereof. Inone non-limiting embodiment of the invention, the metal ion source is ametal salt, such as ferrous chloride, or a carbonate or a hydroxide, innon-restrictive examples, and alternatively a metal complex. Othersuitable, non-limiting sources include ferric chloride, ferrousgluconate, ferrous glucoheptonate, copper chloride, copper acetate,copper sulfate, cuprous chloride, cuprous nitrate, molybdenum acetate,palladium chloride, palladium nitrate, palladium acetate, nickelchloride, nickel acetate, nickel citrate, nickel formate, nickelgluconate, manganese gluconate, manganese glucoheptonate, manganesechloride, zinc glucoheptonate, zinc chloride, aluminum gluconate,aluminum sulfate, aluminum chloride and mixtures thereof.

Additionally, in another non-restrictive embodiment the metal ions maybe complexed or chelated. Suitable sources for complexing and chelatingagents include, but are not limited to, carboxylic acids,aminocarboxylic acids, polyols, alkanolamines, and the like. Examples ofsuitable carboxylic acids include, but are not limited to, fumaric acid,lactic acid, maleic acid, tartaric acid, citric acid, glucaric acid,gluconic acid, and the like. The carboxylic acids may be, and arepreferred to be in salt form, for instance in particular non-restrictiveexamples such as sodium citrate, ammonium citrate, potassium fumarate,sodium gluconate, sodium glucoheptonate, and ammonium lactate. Examplesof suitable aminocarboxylic acids include, but are not limited to,ethylenediaminetetraacetic acid (EDTA), hydroxyethylenediaminetriaceticacid (HEDTA), propylenediaminetetraacetic acid (PDTA),diethylenetriaminepentaacetic acid (DTPA), nitrilotriacetic acid (NTA),iminodisuccinic acid, amino acids, hydroxyethyliminodiacetic acid(HEIDA), and the like. Again, the aminocarboxylic acids may be, and insome cases is preferred to be in salt form, in non-restrictive examplesas tetrasodium EDTA, trisodium NTA, and diammonium dihydrogen EDTA,sodium iminodisuccinate, and disodium HEIDA. Examples of suitablepolyols include, but are not limited to, sorbitol, xylitol, mannitol,and the like. Examples of suitable alkanolamines include, but are notlimited to diethanolamine, triethanolamine, and the like.

It is difficult, if not impossible, to specify with accuracy the amountof the various breaking components that should be added to a particularaqueous fluid gelled with viscoelastic surfactants to sufficiently orfully break the gel, in general. For instance, a number of factorsaffect this proportion, including but not necessarily limited to, theparticular VES used to gel the fluid; the particular metal ion and metalion source used; the particular chelant and particular chelant sourceused; the particular reducing agent and particular reducing agent sourceused; the temperature of the fluid; the downhole pressure of the fluid,the starting pH of the fluid; and the complex interaction of thesevarious factors. Nevertheless, in order to give an approximate feel forthe proportions of the various breaking components to be used in themethod of the invention, approximate ranges will be provided. The amountof elemental metal ion that may be effective in the invention may rangefrom about 0.001 to about 500 ppm, based on the total amount of thefluid, irrespective of the amount of the metal ion source (e.g.gluconate, acetate, chloride, chloride dihydrate, etc. type sources). Inanother non-restrictive version of the invention, the amount ofelemental metal ion may range from about 0.05 to about 400 ppm.

The second optional component is preferably a reducing agent which isfrom either an organic or inorganic source. Suitable reducing agentsources include, but are not necessarily limited to organic acids,organic acid salts, amines, alcohols, reducing sugars, ammoniumcompounds, nitrites, phosphites, sulfites, thiosulfates, thiols,hydrides, tocopherols, tocotrienols, quinones, and the like and mixturesthereof.

In one non-restrictive version of the invention, the optional organicacid used as a reducing agent is selected from the group consisting ofcitric acid, ascorbic acid, dehydroascorbic acid, benzoic acid, gluconicacid, lactic acid, erythorbic acid, formic acid, glycolic acid, oxalicacid, adipic acid, glutaric acid, succinic acid, acetic acid, propionicacid, caproic acid, maleic acid, fumaric acid, tartaric acid, cysteine,methionine, phthalic acid, and the like and mixtures thereof. In anothernon-restrictive embodiment of the invention, the organic acid source maybe in an alkali or alkaline earth metal salt form. The salts of organicacids may include, but are not necessarily limited to citrates,acetates, ascorbates, erythorbates, benzoates, succinates, fumarates,maleates, and gluconates of alkali metals and alkaline earth metals. Asmentioned, the organic acid is preferred to be in the alkali salt formor ammonium salt form in one non-restrictive case, for instance sodiumcitrate, potassium citrate, ammonium citrate, sodium erythorbate, sodiumascorbate, calcium ascorbate, sodium benzoate, sodium phthalate,diammonium phthalate, sodium gluconate, sodium acetate, sodium oxalate,and the like. The amount of organic acid salt that may be effective inthe invention may range from about 1 to about 80 pptg (pounds perthousand gallons) based on the total amount of the aqueous fluid,irrespective of the amount of the organic acid source. In anothernon-restrictive version of the invention, the amount of organic acid mayrange from about 4 to about 40 pptg.

In one non-limiting embodiment of the invention, the reducing agentsource may include, but is not necessarily limited to, sodium nitrite,sodium sulfite, sodium bisulfite, ammonium bisulfite, sodiumthiosulfate, potassium thiosulfate, ammonium thiosulfate, sodiumhydrosulfite, thiourea, hydrazine, sodium hydride, lithium hydride,sodium borohydride, lithium aluminum hydride, dithiothreitol, ethylmercaptan, allyl mercaptan, anthraquinone, naphthoquinone, benzoquinone,glutathione, ethylenethiourea, tocopherols, tocotrienols, and otherreducing agents known in the art may be utilized, and mixtures thereof.The amount of reducing agent that may be effective in the invention mayrange from about 0.2 to about 60 pptg based on the total amount of theaqueous fluid. In another non-restrictive version of the invention, theamount of reducing agent may range from about 0.5 to about 40 pptg.

In one non-limiting embodiment of the invention, the reducing sugars areselected from the group consisting of mono-, and disaccharides, andmixtures thereof. In another non-restrictive embodiment of theinvention, the reducing sugars may include, but are not necessarilylimited to glucose, fructose, mannose, galactose, maltose, lactose andxylose. The amount of reducing sugar that may be effective in theinvention may range from about 2 to about 120 pptg based on the totalamount of the aqueous fluid, irrespective of the amount of the reducingsugar source. In another non-restrictive version of the invention, theamount of reducing sugar of may range from about 5 to about 50 pptg.

In one non-limiting embodiment of the invention, the optionalhydrogenation-dehydrogenation agent besides water is selected from thegroup consisting of aldehydes, ketones, alcohols, glycols, sugaralcohols, carbonates, phosphates, borohydrides, ammonium compounds, andthe like and mixtures thereof. In another non-restrictive embodiment ofthe invention, the optional hydrogenation-dehydrogenation agent sourcemay include, but are not necessarily limited to, acetaldehyde,propionaldehyde, butyraldehyde, cinnamaldehyde, acetone, methyl ethylketone, methyl isopropyl ketone, glycine, lysine, arginine, glutamine,ammonia, ammonium chloride, urea, tetramethylammonium chloride, choline,hexamethylene diamine, triethylene glycol diamine, methanol,isopropanol, ethanol, hexanol, glycerol, propylene glycol, tripropyleneglycol, diethylene glycol, disodium hydrogen phosphate, sodiumdihydrogen phosphate, ammonium dihydrogen phosphate, boric acid, sodiumborate, sodium-calcium borate, sodium carbonate, sodium bicarbonate,sodium sesquicarbonate, sodium borohydride, sodium hydride, lithiumaluminum hydride, and the like. The amount ofhydrogenation-dehydrogenation agent that may be effective in theinvention may range from about 1 to about 100 pptg based on the totalamount of the aqueous fluid, irrespective of the amount of thehydrogenation-dehydrogenation agent. In another non-restrictive versionof the invention, the amount of hydrogenation-dehydrogenation agent mayrange from about 5 to about 50 pptg.

Optionally, one or more suitable conventional or future oxidizing agentsuseful for catalytic redox reactions with viscoelastic surfactantmolecules may also be employed in the breaking composition of thisinvention. Appropriate oxidizers include, but are not necessarilylimited to, alkali metals and alkaline earth metals of persulfates,percarbonates, perborates, peroxides, hydroperoxides, bromates,bromides, hypochlorites, chlorites, perchlorates, periodates,permanganates, perphosphates, hydrogen peroxide, and the like andmixtures thereof. The amount of oxidizer that may be effective in theinvention may range from about 0.5 to about 100 pptg based on the totalamount of the aqueous fluid. In another non-restrictive version of theinvention, the amount of oxidizer may range from about 2 to about 50pptg.

An optional additional component of the breaking composition of thisinvention is an organic compound that slowly hydrolyzes upon fluidheating into a Brønsted-Lowry acid as an organic hydrogenation source.These organic compounds, which may typically include organic acids, mayinclude, but are not necessarily limited to, citric acid esters, fumaricacid esters, acetic acid esters, and the like. The specific organiccompounds include, but are not limited to, ethyl acetate, ethylacetoacetate, triethyl citrate, tributyl citrate, and diethyl fumarate.The amount of organic acid that may be effective in the invention mayrange from about 0.2 to about 8 gptg based on the total amount of theaqueous fluid. In another non-restrictive version of the invention, theamount of organic acid may range from about 0.5 to about 4 gptg.

Any suitable mixing apparatus may be used for this procedure. In thecase of batch mixing, the VES and the aqueous fluid are blended for aperiod of time sufficient to form a gelled or viscosified solution. TheVES that is useful in the present invention can be any of the VESsystems that are familiar to those in the well service industry, and mayinclude, but are not limited to, amidoamine oxides, amines, amine salts,quaternary ammonium compounds, amine oxides, ethoxylated fatty amines,methyl ester sulfonates, betaines, modified betaines, sulfosuccinates,mixtures thereof and the like. Suitable amines, amine salts, quaternaryammonium salts, amidoamine oxides, and other surfactants are describedin U.S. Pat. Nos. 5,964,295; 5,979,555; 6,239,183 and 7,261,160,incorporated herein by reference.

Viscoelastic surfactants improve the fracturing (frac) fluid performancethrough the use of a polymer-free system. These systems offer improvedviscosity breaking, higher sand transport capability, are more easilyrecovered after treatment, and are relatively non-damaging to thereservoir. The systems are also more easily mixed “on the fly” in fieldoperations and do not require numerous co-additives in the fluid system,as do some prior systems.

The viscoelastic surfactants suitable for use in this invention include,but are not necessarily limited to, non-ionic, cationic, amphoteric, andzwitterionic surfactants. Specific examples of zwitterionic/amphotericsurfactants include, but are not necessarily limited to, dihydroxylalkyl glycinate, alkyl ampho acetate or propionate, alkyl betaine, alkylamidopropyl betaine and alkylimino mono- or di-propionates derived fromcertain waxes, fats and oils. Quaternary amine surfactants are typicallycationic, and the betaines are typically zwitterionic. The thickeningagent may be used in conjunction with an inorganic water-soluble salt ororganic additive such as phthalic acid, salicylic acid or their salts.

Some non-ionic fluids are inherently less damaging to the producingformations than cationic fluid types, and are more efficacious per poundthan anionic gelling agents. Amine oxide viscoelastic surfactants havethe potential to offer more gelling power per pound, making it lessexpensive than other fluids of this type.

The amine oxide gelling agents RN⁺(R′)₂O⁻ may have the followingstructure (I):

where R is an alkyl or alkylamido group averaging from about 8 to 24carbon atoms and R′ are independently alkyl groups averaging from about1 to 6 carbon atoms. In one non-limiting embodiment, R is an alkyl oralkylamido group averaging from about 8 to 16 carbon atoms and R′ areindependently alkyl groups averaging from about 2 to 3 carbon atoms. Inan alternate, non-restrictive embodiment, the amidoamine oxide gellingagent is Akzo Nobel's Aromox APA-T formulation, which should beunderstood as a dipropylamine oxide since both R′ groups are propyl.

Materials sold under U.S. Pat. No. 5,964,295 include ClearFRAC™, whichmay also comprise greater than 10% of a glycol. One preferred VES is anamine oxide. As noted, a particularly preferred amine oxide is APA-T,sold by Baker Oil Tools as SurFRAQ™ VES. SurFRAQ is a VES liquid productthat is 50% APA-T and 40% propylene glycol. These viscoelasticsurfactants are capable of gelling aqueous solutions to form a gelledbase fluid. The additives of this invention may also be used in DiamondFRAQ™ which is a VES system, similar to SurFRAQ, sold by Baker OilTools.

The invention covers commonly known materials as Aromox APA-Tmanufactured by Akzo Nobel and other known viscoelastic surfactantgelling agents common to stimulation treatment of subterraneanformations.

The amount of VES included in the fracturing fluid depends on at leasttwo factors. One involves generating enough viscosity to control therate of fluid leak off into the pores of the fracture, and the secondinvolves creating a viscosity high enough to keep the proppant particlessuspended therein during the fluid injecting step, in the non-limitingcase of a fracturing fluid. Thus, depending on the application, the VESis added to the aqueous fluid in concentrations ranging from about 0.5to 25% by volume, alternatively up to about 12 vol % of the totalaqueous fluid (from about 5 to 120 gallons per thousand gallons (gptg)).In another non-limiting embodiment, the range for the present inventionis from about 1.0 to about 6.0% by volume VES product. In an alternate,non-restrictive form of the invention, the amount of VES ranges from 2to about 10 volume %.

It is expected that the breaking compositions of this invention can beused to reduce the viscosity of a VES-gelled aqueous fluid regardless ofhow the VES-gelled fluid is ultimately utilized. For instance, theviscosity breaking compositions could be used in all VES applicationsincluding, but not limited to, VES-gelled friction reducers, VESviscosifiers for loss circulation pills, fracturing fluids, gravel packfluids, viscosifiers used as diverters in acidizing, VES viscosifiersused to clean up drilling mud filter cake, remedial clean-up of fluidsafter a VES treatment (post-VES treatment), and the like.

A value of the invention is that a fracturing or other fluid can bedesigned to have enhanced breaking characteristics. Importantly, betterclean-up of the VES fluid from the fracture and wellbore can be achievedthereby. Better clean-up of the VES directly influences the success ofthe fracture treatment, which is an enhancement of the well'shydrocarbon productivity.

In order to practice the method of the invention, an aqueous fracturingfluid, as a non-limiting example, is first prepared by blending a VESinto an aqueous fluid. The aqueous fluid could be, for example, water,brine, aqueous-based foams or water-alcohol mixtures. Any suitablemixing apparatus may be used for this procedure. In the case of batchmixing, the VES and the aqueous fluid are blended for a period of timesufficient to form a gelled or viscosified solution. Alternatively, thebreaking composition of this invention may be added separately.

Propping agents are typically added to the base fluid after the additionof the VES. Propping agents include, but are not limited to, forinstance, quartz sand grains, glass and ceramic beads, bauxite grains,walnut shell fragments, aluminum pellets, nylon pellets, and the like.The propping agents are normally used in concentrations between about 1to 14 pounds per gallon (120-1700 kg/m³) of fracturing fluidcomposition, but higher or lower concentrations can be used as thefracture design required. The base fluid can also contain otherconventional additives common to the well service industry such as waterwetting surfactants, non-emulsifiers and the like. As noted, in thisinvention, the base fluid can also contain other non-conventionaladditives which can contribute to the breaking action of the VES fluid,and which are added for that purpose.

Any or all of the above metal ion sources and/or organic and/orinorganic redox agent sources and/or hydrogenation-dehydrogenation agentsources, chelating agents, organic acids, etc. may be provided in anextended release form such as encapsulation by polymer or otherwise,pelletization with binder compounds, absorbed or some other method oflayering on a microscopic particle or porous substrate, and/or acombination thereof. Specifically, the sources may be encapsulated topermit slow or timed release thereof. In non-limiting examples, thecoating material may slowly dissolve or be removed by any conventionalmechanism, or the coating could have very small holes or perforationstherein for the bio-products within to diffuse through slowly. Forinstance, polymer encapsulation coatings such as used in fertilizertechnology available from Scotts Company, specifically POLY-S® productcoating technology, or polymer encapsulation coating technology fromFritz Industries could possibly be adapted to the methods of thisinvention. The sources could also be absorbed onto zeolites, such asZeolite A, Zeolite 13X, Zeolite DB-2 (available from PQ Corporation,Valley Forge, Pa.) or Zeolites Na-SKS5, Na-SKS6, Na-SKS7, Na-SKS9,Na-SKS10, and Na-SKS13, (available from Hoechst Aktiengesellschaft, nowan affiliate of Aventis S.A.), and other porous solid substrates such asMICROSPONGE™ (available from Advanced Polymer Systems, Redwood, Calif.)and cationic exchange materials such as bentonite clay or microscopicparticles such as carbon nanotubes or buckminster fullerenes. Further,the component sources may be both absorbed into and onto poroussubstrates and then encapsulated or coated, as described above.

In a typical fracturing operation, the fracturing fluid of the inventionis pumped at a rate sufficient to initiate and propagate a fracture inthe formation and to place propping agents into the fracture. A typicalfracturing treatment would be conducted by mixing a 20.0 to 60.0gallon/1000 gal water (volume/volume—the same values may be used withany SI volume unit, e.g. 60.0 liters/1000 liters) amine oxide VES, suchas SurFRAQ, in a 2% (w/v) (166 lb/1000 gal, 19.9 kg/m³) KCl solution ata pH ranging from about 6.0 to about 8.0. The breaking components areadded after the VES addition, or in a separate step after the fracturingoperation is complete.

In one embodiment of the invention, the method of the invention ispracticed in the absence of gel-forming polymers and/or gels or aqueousfluid having their viscosities enhanced by polymers.

The present invention will be explained in further detail in thefollowing non-limiting Examples that are only designed to additionallyillustrate the invention but not narrow the scope thereof.

General Procedure for Examples 1-8

To a blender were added tap water, 3 wt % KCl, followed by 3 vol %viscoelastic surfactant (WG-3L—Aromox® APA-T from Akzo Nobel). Theblender was used to mix the components on a very slow speed, to preventfoaming, for about 15 minutes to viscosify the VES fluid. Mixed sampleswere then placed into plastic bottles. Various components singly ortogether, in various concentrations, were then added to each sample, andthe sample was shaken vigorously for 60 seconds. The samples were placedin a water bath at the indicated temperature and visually observed every30 minutes for viscosity reduction difference between the samples. Sincea goal of the research was to find a relatively rapid gel breakingcomposition, samples were only observed for 24 hours or less.

Viscosity reduction can be visually detected. Shaking the samples andcomparing the elasticity of gel and rate of air bubbles rising out ofthe fluid can be used to estimate the amount of viscosity reductionobserved. Measurements using a Fann 35 rheometer at 100 rpm can also beused to acquire quantitative viscosity reduction of each sample.

Example 1

The results of Example 1 comparing no Fe⁺² with 200 ppm Fe⁺² and 400 ppmFe⁺² are shown in FIG. 1. The source of the Fe⁺² was ferrous chloride.No other breaker components were added. This Example shows that atransition metal alone can readily break a VES-gelled fluid, eitherslowly or very quickly proportional to the amount of metal used. Unlessotherwise noted, Examples 7-14 use a base fluid composition of tap waterincluding 3% bw KCl and 4% bv WG-3L viscoelastic surfactant.

Example 2

The results of Example 2 shown in FIG. 2 demonstrates that breaking ratecan be enhanced by use of a chelant, and that the breaking profile(viscosity over time) can be altered and also enhanced by the type ofchelant used. Test results with AM2-45 present show some chelants cangive stable initial viscosity followed by very sharp breaking rate(viscosity reduction). The DISSOLVINE AM2-45 is a EDTA-(NH₄)₂H₂ chelantavailable from Akzo Nobel. The data within FIG. 2 show thatmetal-mediated viscosity reduction can be enhanced by the addition of acombination of chelants or use of a one particular type.

Example 3

The data from Example 3 plotted in FIG. 3 demonstrates that less amountof metal is required for VES viscosity break when activated by areducing agent such sodium ascorbate. Note that the amount of Fe⁺² wasdecreased by a factor of 10 when the sodium ascorbate was used.

Example 4

FIG. 4 presents the results from Example 4 showing synergistic breakingof VES occurs when combining with the transition metal a chelant(AM2-45) and a reducing agent (sodium ascorbate), as shown by the lineof short dashes demonstrating a very quick break.

Example 5

FIG. 5 shows the results of Example 5 that once an adequate amount ofmetal and chelant is present the amount of reducing agent can beoptimized. Note how once enough reducing agent (sodium ascorbate) ispresent that no increase in breaking rate occurs with extra amountsadded (i.e. no appreciable difference between 10.5 pptg (1.3 kg/m³), 14pptg (1.7 kg/m³) and 17.5 pptg (2.1 kg/m³)).

GBW-197L is an aqueous solution composed of four transition metals withthree chelants. The total metal content is about 5 wt %.

Example 6

The data from Example 6 are graphed in FIG. 6 and demonstrate that withadequate amounts of chelant and reducing agent present the viscositybreak time of a VES fluid can be adjusted by varying the amount ofmetal. Note how with synergistic combination of chelant and reducingagent that very little metal is required to obtain fast gelbreaks—amounts of 2, 1, and even 0.5 ppm Fe⁺² (from ferrous chloride).

Example 7

FIG. 7 presents the data from Example 7 which show that use of more thanone metal source can be used to enhance the rate of VES-gel break. Notehow the select ratio and amount of metals tested shows similar enhancedbreaking rates, with each metal combination showing some distinction inthe particular breaking profile.

In this Example, the Base Fluid Composition was 3% bw KCl, 3% bv WG-3L,10 pptg Ascorbate, with the balance as tap water. The Fe⁺³ source wasD-Fe-6 (Akzo Nobel Dissolvine DTPA-Fe⁺³(NH₄)₂ product). The ascorbatewas sodium ascorbate. The Mn⁺² was from E-Mn-6 (Akzo Nobel DissolvineMnK₂ EDTA product). The Cu⁺² was from E-Cu-15 (Akzo Nobel DissolvineCuNa₂ EDTA product). The Co+3 was from reagent CoCl₃ complexed with AkzoNobel Dissolvine AM2-45. The Ni⁺² was from reagent NiCl₂ complexed withAkzo Nobel Dissolvine AM2-45. The Zn⁺² was from reagent ZnCl₂ complexedwith Akzo Nobel Dissolvine AM2-45.

Example 8

FIG. 8 presents the data from Example 8 showing that an oxidizer canfurther activate a metal and reducing agent combination for improved VESgel breaking. The oxidizer was sodium persulfate.

Overall, the Examples 1-8 (FIGS. 1-8) show a single transition metal canbe used to break VES gel, and that the rate of breaking can be enhancedby use of one of several optional activating agents. Of particularutility is the synergistic combination of a chelant and reducing agentwith one or more transition metals. Multiple agents can be used with ametal to control when and how fast a VES gelled-fluid will break. In allmultiple agent combinations the rate of break can be varied by adjustingthe amount of metal or metals used providing significant control overthe breaking procedure.

A summary of what has been discovered herein relating to VES-gelbreaking technology includes, but is not necessarily limited to:

-   -   a. that a metal ion alone may be used;    -   b. that metal-mediated VES-gel breaking can be enhanced by use        of a chelating agent;    -   c. that metal-mediated VES-gel breaking can be additionally        enhanced by use of a reducing agent;    -   d. that metal-mediated VES-gel breaking can be substantially        enhanced by use of a combination of a chelating agent and a        reducing agent; and    -   e. that metal-mediated VES-gel breaking can be further enhanced        by use of two or more transition metal agents.

As previously discussed, it is possible that the breaker system hereinworks by one or more redox reactions. These reactions may or may notexplain the breaking mechanisms at work in the technology describedherein, and the inventors do not wish to be limited to any particularexplanation. Further, it is possible that two or more breaking methodsparticipate at the same time or sequentially in the VESalteration-degradation herein. The reactions that alter the VES compoundto a non-VES compound are viewed as the primary reactions that mayoccur. Secondary reactions with the non-VES compounds generated mayoccur under conditions including, but not necessarily limited to:

-   -   a. when more than one metal ion is present;    -   b. when more than one chelant is present;    -   c. when more than one reducing agent is present;    -   d. when pH buffers are used; and    -   e. when other agents are present that may influence or        participate in any additional alteration of the non-VES        compounds

In the foregoing specification, the invention has been described withreference to specific embodiments thereof, and has been demonstrated aseffective in providing methods and compositions for a VES fracturingfluid breaker mechanism. However, it will be evident that variousmodifications and changes can be made thereto without departing from thebroader spirit or scope of the invention as set forth in the appendedclaims. Accordingly, the specification is to be regarded in anillustrative rather than a restrictive sense. For example, specificcombinations of viscoelastic surfactants, metal ions, metal ion sources,organic and inorganic redox agents, organic and inorganic redox agentsources, organic and inorganic hydrogenation-dehydrogenation agents,organic and inorganic hydrogenation-dehydrogenation agent sources,chelating agents, hydrolyzing organic acids, and other componentsfalling within the claimed parameters, but not specifically identifiedor tried in a particular composition or fluid, are anticipated to bewithin the scope of this invention.

1. A method for breaking viscosity of aqueous fluids gelled with aviscoelastic surfactant (VES) comprising adding to an aqueous fluidgelled with at least one VES in an amount effective to increase theviscosity of the aqueous fluid, a composition in an amount effective toreduce the viscosity of the gelled aqueous fluid, where the compositioncomprises from about 0.01 to about 300 ppm, based on the total fluid ofat least one transition metal ion source, and where the VES is selectedfrom the group consisting of amines, amine salts, quaternary ammoniumcompounds, amine oxides, ethoxylated fatty amines, methyl estersulfonates, betaines, modified betaines, sulfosuccinates, and mixturesthereof.
 2. The method of claim 1 where the composition furthercomprises at least one chelating agent.
 3. The method of claim 2 wherethe composition further comprises at least one reducing agent source. 4.The method of claim 3 where the reducing agent source is selected fromthe group consisting of erythorbates, dehydroascorbates, citrates,ascorbates, sulfites, thiols, and alkali metal, alkaline earth metal andammonium salts thereof.
 5. The method of claim 2 where the chelatingagent is selected from the group consisting of carboxylic acids,aminocarboxylic acids, polyols, alkanolamines, and combinations thereof.6. The method of claim 2 where the amount of chelating agent ranges fromabout 0.1 to about 50 pptg (about 0.012 to about 6 kg/m³), based on thetotal fluid.
 7. The method of claim 1 where the transition metal ionsource is a transition metal salt or transition metal complex.
 8. Themethod of claim 7 where the transition metal in the transition metalsalt or transition metal complex is a transition metal selected from thegroup consisting of Groups VA, VIA, VIIA, VIIIA, IB, IIB, IIIB, and IVBof the Periodic Table (previous IUPAC American Group notation).
 9. Themethod of claim 1 where the composition further comprises at least twotransition metal ion sources.
 10. An aqueous fluid comprising: water; atleast one viscoelastic surfactant (VES) in an amount effective toincrease the viscosity of the aqueous fluid, where the VES is selectedfrom the group consisting of amines, amine salts, quaternary ammoniumcompounds, amine oxides, ethoxylated fatty amines, methyl estersulfonates, betaines, modified betaines, sulfosuccinates, and mixturesthereof; and a composition in an amount from about 0.01 to about 300 ppmbased on the total fluid, to reduce the viscosity of the gelled aqueousfluid, where the composition comprises at least one transition metal ionsource.
 11. The aqueous fluid of claim 10 where the composition furthercomprises at least one chelating agent.
 12. The aqueous fluid of claim11 where the composition further comprises at least one reducing agentsource.
 13. The aqueous fluid of claim 12 where the reducing agentsource is selected from the group consisting of erythorbates,dehydroascorbates, citrates, ascorbates, sulfites, thiols, and alkalimetal, alkaline earth metal and ammonium salts thereof.
 14. The aqueousfluid of claim 11 where the amount of chelating agent ranges from about0.1 to about 50 pptg (about 0.012 to about 6 kg/m³), based on the totalfluid.
 15. The aqueous fluid of claim 10 where the transition metal ionsource is a transition metal salt or transition metal complex.
 16. Theaqueous fluid of claim 15 where the transition metal in the transitionmetal salt or transition metal complex is a transition metal selectedfrom the group consisting of Groups VA, VIA, VIIA, VIIIA, IB, IIB, IIIB,and IVB of the Periodic Table (previous IUPAC American Group notation).17. The aqueous fluid of claim 10 where the composition furthercomprises at least two transition metal ion sources.
 18. The aqueousfluid of claim 10 where the chelating agent is selected from the groupconsisting of carboxylic acids, aminocarboxylic acids, polyols,alkanolamines, and combinations thereof.
 19. An aqueous fluidcomprising: water; at least one viscoelastic surfactant (VES) in anamount effective to increase the viscosity of the aqueous fluid, wherethe VES is selected from the group consisting of amines, amine salts,quaternary ammonium compounds, amine oxides, ethoxylated fatty amines,methyl ester sulfonates, betaines, modified betaines, sulfosuccinates,and mixtures thereof; and a composition in an amount effective to reducethe viscosity of the gelled aqueous fluid, where the compositioncomprises: from about 0.01 to about 300 ppm, based on the total fluid ofat least one transition metal ion source; at least one chelating agent;and at least one reducing agent source; where the composition reducesthe viscosity of the gelled aqueous fluid by a mechanism selected fromthe group consisting of disaggregating a micelle structure of the VES,rearranging a micelle structure of the VES, chemically altering aneffective amount of the VES, and combinations thereof.
 20. The aqueousfluid of claim 19 where the transition metal ion source is a transitionmetal salt or transition metal complex.
 21. The aqueous fluid of claim19 where the transition metal in the transition metal salt or transitionmetal complex is a transition metal selected from the group consistingof Groups VA, VIA, VIIA, VIIIA, IB, IIB, IIIB, and IVB of the PeriodicTable (previous IUPAC American Group notation).
 22. The aqueous fluid ofclaim 19 where the chelating agent is selected from the group consistingof carboxylic acids, aminocarboxylic acids, polyols, alkanolamines, andcombinations thereof.
 23. The aqueous fluid of claim 19 where thereducing agent source is an organic acid salt selected from the groupconsisting of erythorbates, dehydroascorbates, citrates, ascorbates,sulfites, thiols, and alkali metal, alkaline earth metal and ammoniumsalts thereof.
 24. The aqueous fluid of claim 19 where the amount ofreducing agent source ranges from about 5 to about 50 pptg (about 0.6 toabout 6 kg/m³), based on the total fluid; and the amount of chelatingagent ranges from about 0.1 to about 50 pptg (about 0.012 to about 6kg/m³), based on the total fluid.